When we are dealing with an engineering drawing, if we find that certain dimensions do not have specified tolerances, they are usually handled according to specific standards. These standards are designed to ensure that the manufactured parts meet requirements in both function and quality.
In most cases, dimensions with unspecified tolerances on a drawing rely on one of the following standards:
ISO 2768: This is an international standard applicable to dimensions with unspecified tolerances. It is divided into several tolerance classes (such as fine, medium, and coarse), and the appropriate class is selected based on the part's intended use and manufacturing process.
ANSI/ASME Y14.5: This is a standard established by the American Society of Mechanical Engineers, widely adopted in the United States and many other countries. It details the specification methods for Geometric Dimensioning and Tolerancing (GD&T), and dimensions with unspecified tolerances usually have default tolerance classes.
Internal Company Standards: Many large companies establish their own internal standards based on international or national standards, combined with their own production capabilities and quality control requirements. For example, some companies may have specific internal documents or manuals that define default tolerances for different size ranges.
Contracts or Design Specifications: In some projects, contract documents or design specifications may explicitly specify how unspecified tolerances are to be handled. If so, we need to strictly follow the provisions of these documents.
When we find dimensions with unspecified tolerances on a drawing, we must first check if there are any supplementary notes, technical requirements, or reference documents. If there are no explicit instructions, we will handle them according to applicable standards or company regulations. This ensures the consistency and manufacturability of all parts, avoiding functional failures or assembly issues caused by improper tolerances.
Specify tightest tolerance below, making sure all dimensions with tighter tolerances than ISO 2768-1 are clearly indicated on your technical drawing.
link:CNC Machining DFM Guidelines & Size Capabilities Quick Reference
